Ronda Pindzola has a Ph.D. degree in Bio-Psychology and did her post doctoral fellowships at Ohio State University in anatomy and Case Western Reserve University in neuroscience. Since then she has moved into the field of clinical research and for the last four years has been working with Drs. Yonas and Nemoto on studies involving patients with SAH or carotid stenosis/occlusion. Her long term objectives involve the study of factors which influence the risk for stroke for the purpose of improving treatment therapies for patients with cerebrovascular disease. Although Dr. Pindzola has a solid background in research, she needs further training in aspects of research that are specific to clinical studies such as interpretation of diagnostic studies, epidemiology and statistics. Dr. Nemoto and Dr. Yonas are extremely well suited to function as her mentor and co-mentor due to their different backgrounds which culminated in clinical research of cerebrovascular disease. Dr. Pindzola also has a multidisciplinary team of investigators including neuroradiologists, neurosurgeons, vascular surgeons and an epidemiologist/statistician to help her recruit patients in addition to guidance in other aspects of the study. Several studies have shown that hemodynamic compromise and high risk for stroke can be identified by transcranial Doppler ultrasonography (TCD) and stable Xenon-enhanced computed tomography (Xe/CT) cerebral blood flow (CBF) measurements. The purpose of this study is to define the relationship between Xe/CT CBF and TCD and near infrared spectroscopy (NIRS) measurements in order to best identify the patients with compromised cerebrovascular reserves. The relationship of the degree of stenosis to cerebrovascular reserves will also be studied since cerebrovascular reserves may be more indicative of risk for stroke than degree of stenosis. The hypothesis is that the measurement of cerebrovascular reserves with Xe/CT CBF methods, will allow the investigators to best identify patients that have severely compromised reserves and thus are at greatest risk for subsequent stroke. Dr. Pindzola hopes to extend this study in the future to asymptomatic patients. Several recent studies have shown that patients with asymptomatic carotid stenosis benefit from carotid endarterectomy. However, the ACAS trial found that 17 patients needed to be operated on to prevent one stroke or death over five years. In order to derive this benefit of stroke prevention many unnecessary surgeries would have to be done. The asymptomatic patients who are at risk for stroke could be identified by their blood flow reserves. First the applicant needs to determine the most accurate method of determining compromised reserves. Then Dr. Pindzola plans to design a study to assess risk for stroke in asymptomatic patients based on cerebrovascular reserve measurements.